The use of nitrate, bacteria and fluorescent tracers to characterize groundwater recharge and contamination in a karst catchment, Chongqing, ChinaUtilisation de nitrates, de bactéries et traceurs fluorescents pour caractériser la recharge et la contamination d’un captage karstique, Chongqing, ChineEl uso de nitratos, bacterias y trazadores fluorescentes para caracterizar la recarga y la contaminación de agua subterránea en una cuenca kárstica, Chongqing, China利用硝酸盐、细菌和荧光示踪剂刻画中国重庆某喀斯特流域的地下水补给和污染Utilização do nitrato, de bactérias e de traçadores fluorescentes na caracterização da recarga e da contaminação da água subterrânea numa bacia cársica, Chongqing, China

The Qingmuguan subterranean river system is located in the suburb of Chongqing, China, and it is the drinking water source that local people downstream rely on. The study aims to provide a scientific basis for groundwater protection in that area, using a hydrogeological framework, tracer tests, hydrological online monitoring, and hydrochemical and microbiological investigation, including heterotrophic plate count (HPC) and the analysis of denitrifying bacteria (DNB) and nitrobacteria (NB). The tracer tests proved simple and direct connections between two important sinkholes and the main springs, and also proved that the underground flows here are fast and turbulent. DNB and NB analyses revealed that the main recharge to the underground river in the dry season is the soil-leached water passing through the fissures of the epikarst, while in the rainy season, it is the surface water flow through sinkholes. The hydrochemical and microbiological data confirmed the notable impact of agriculture and sewage on the spring water quality. In the future, groundwater protection here should focus on targeted vulnerability mapping that yields different protection strategies for different seasons.